In radio systems, various frequency synthesis methods are used which employ, for instance, a reference signal source or a control signal by means of which one or more frequencies are generated. In a digital frequency synthesis, commonly used methods include DDS (Direct Digital Synthesis) and NCO (Numerically Controlled Oscillator) methods. Particularly in a radio system using frequency hopping, the synthesizer which performs the synthesis must be capable of becoming tuned quickly in small frequency steps over the entire frequency band. Synthesis methods are used together with various modulation methods.
Converting digital frequency information into analog in a DA converter is one of the drawbacks of the frequency synthesis. The conversion of a signal into analog has contributed to the generation of interfering signals in the output of the synthesis arrangement. Interfering signals have also been generated in an AD-converted signal. The interfering signals have caused interference peaks in the spectrum of the output signal of the synthesizer arrangement. If the interfering signals have exceeded the limits for interfering signals mentioned in the recommendations, the operation of some parts of the radio system has been impaired or possibly hindered altogether.
In frequency synthesis arrangements, various methods are used for reducing interfering signals in the output signal. A known method employs a digital frequency word by which the frequency of a signal obtained from the output of the arrangement is determined. In the method, the periodicity of the signal is broken, which results in fewer potential interfering signals in the output signal. The signal is broken at every clock cycle. The value of a breaking signal varies in such a manner that at every other clock cycle the value is a logical zero, and at every other clock cycle the value is a logical one.
The problem of the method described above is that the signal breakage has not been sufficiently efficient. Consequently, the output signal includes interfering signals even after the breakage.